1. Field of the Invention
The present invention relates to a fixing apparatus to be used on electronic image forming apparatuses such as copying machines, printers, and facsimiles, in particular to a belt type fixing apparatus.
2. Description of the Related Arts
The fixing apparatus is intended to fix unfixed a toner image recorded on a recording medium such as recording paper and an OHP sheet, and is typically a thermal roller type apparatus.
This thermal roller type fixing apparatus comprises a heating roller and a pressing roller that presses on said heating roller, and the recording medium is passed through a nipping area consisting of these two rollers where the unfixed toner image is fixed on the recording medium due to heat and pressure.
In the meanwhile, a fixing apparatus using a belt has been developed in recent years (e.g., Kokai Hei 6-318001). This belt type fixing apparatus has an advantage that it can be heated more quickly thus providing a quick printing feature, because of its low heat capacity.
Since this belt type fixing apparatus comprises a belt and a contacting member (pressuring roller) that forms a nipping area by means of contacting the belt surface, this nipping area has a wider surface for supporting the recording member than the heating roller type and a longer period of heating by the belt, thus supplying more heat to the toner on the recording member.
As a consequence, it is possible to supply a sufficient amount of heat for fixing the toner image on the recording medium even if a low belt temperature is used, which also contributes to a lower toner temperature after it has passed the nipping area. It also alleviates the deterioration of the toner coagulation power, i.e., prevents excess melting, minimizes the consumption of toner releasing agents, which are indispensable for fixing apparatuses used in full-color image forming apparatuses, and makes it possible to realize compact and simpler releasing agent coating mechanisms.
However, a problem with this belt type fixing apparatus is that, in contrast to the heat roller type where the entire roller is heated, only a portion of the belt is heated due to the fact one of the rollers on which the belt is wound around is used as a heat source.
Moreover, it is difficult to heat the entire belt to a uniform temperature even if a low heat capacity belt is used, as the belt is being heated while it is circulated. Temperature fluctuation in the belt transfer direction is more conspicuous during the initial period of heating.
Image luster fluctuation and deterioration of image quality as a result can be a problem caused by sending recording media into the nipping area having such a temperature fluctuation.